Membrane electrode assemblies and fuel cells with long lifetime

ABSTRACT

The present invention relates to improved membrane electrode assemblies and fuel cells with long lifetime, comprising two electrochemically active electrodes separated by a polymer electrolyte membrane based on polyoxazoles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit (under 35 USC 119(e)) of U.S.Provisional Application 61/475,239, filed Apr. 14, 2011, which isincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to improved membrane electrode assembliesand fuel cells with long lifetime, comprising two electrochemicallyactive electrodes separated by a polymer electrolyte membrane.

In polymer electrolyte membrane (PEM) fuel cells, the proton-conductingmembranes used nowadays are almost exclusively sulfonic acid-modifiedpolymers. Predominantly perfluorinated polymers are employed. Aprominent example thereof is Nafion™ from DuPont de Nemours, Wilmington,USA. For proton conduction, a relatively high water content in themembrane is required, which is typically 4-20 molecules of water persulfonic acid group. The water content needed, but also the stability ofthe polymer in conjunction with acidic water and the hydrogen and oxygenreaction gases, limits the operating temperature of the PEM fuel cellstacks to 80-100° C. Higher operating temperatures cannot be achievedwithout loss of performance of the fuel cell. At temperatures above thedew point of water for a given pressure level, the membrane dries outcompletely, and the fuel cell no longer supplies any electrical energysince the resistance of the membrane rises to such high values thatthere is no longer any significant current flow.

For system-related reasons, however, higher operating temperatures than100° C. in the fuel cell are desirable. The activity of thenoble-metal-based catalysts present in the membrane electrode assembly(MEA) is much better at high operating temperatures.

More particularly, in the case of use of what are called reformates fromhydrocarbons, distinct amounts of carbon monoxide are present in thereformer gas and typically have to be removed by a costly andinconvenient gas processing or gas cleaning operation. At high operatingtemperatures, the tolerance of the catalysts to the CO impurities rises.

In addition, heat arises in the operation of fuel cells. However,cooling of these systems to below 80° C. can be very costly andinconvenient. According to the power released, the cooling apparatus canbe made much simpler. This means that, in fuel cell systems which areoperated at temperatures above 100° C., the waste heat can be utilizedmuch better and hence the fuel cell system efficiency can be enhanced.

In order to attain these temperatures, membranes with novel conductivitymechanisms are generally used. One approach to doing this is the use ofmembranes which exhibit ionic conductivity without the use of water. Afirst promising development in this direction is described in thepublication WO 96/13872. A further high-temperature fuel cell isdisclosed in the publication JP-A-2001-196082.

In addition, WO 02/088219 discloses a second generation ofhigh-temperature fuel cell based on polyazoles, which are produced bycondensation polymerization in polyphosphoric acid (PPA) and partialhydrolysis of the same reaction mixture. These proton-conducting polymermembranes exhibit improved properties compared to the membranes knownfrom WO 96/13872. Nevertheless, these membranes too can still beimproved for long-term operation in a high-temperature fuel cell.Especially in the case of sustained use temperatures of 160-180° C. andfrequent startup and shutdown of the fuel cell, degradation or aging ofthe membrane cannot be ruled out. Under some circumstances, thisdegradation can lead to an irreversible failure of the membraneelectrode assembly.

A SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved proton-conducting polymer membrane which does not have theaforementioned problem and thus leads to an improved MEA.

The inventive membrane or MEA which comprises such a membrane hasespecially the following properties:

-   -   The cells in the case of operation at temperatures above 100° C.        should exhibit a long lifetime.    -   The individual cells should exhibit constant or improved        performance at temperatures above 100° C. over a long period.    -   At the same time, the fuel cells should have, after long        operating time, a high zero-load voltage and low gas crossover.    -   The fuel cells should be usable especially at operating        temperatures above 100° C. and not need any additional fuel gas        moistening. More particularly, the membrane electrode assemblies        should be able to withstand permanent or changing pressure        differences between anode and cathode.    -   In addition, it was therefore an object of the present invention        to provide a membrane electrode assembly which can be produced        in a simple and inexpensive manner. At the same time, more        particularly, a minimum amount of expensive materials was to be        used.    -   More particularly, the fuel cell even after a long time should        have a high voltage and be operable at low stoichiometry.    -   More particularly, the MEA should be robust to different        operating conditions (T, p, geometry, etc.) in order to increase        general reliability.    -   Furthermore, expensive noble metal, especially platinum metals,        should be exploited very effectively.

We have now found that a specific proton-conducting membrane based onpolybenzoxazoles can be obtained, which fulfills the aforementionedprofile of requirements.

These objects are achieved by the proton-conducting membrane having allfeatures of claim 1, and the preferred embodiments indicated in thedependent claims, and a membrane electrode assembly comprising aninventive proton-conducting membrane.

The present invention provides a proton-conducting polymer membranebased on polybenzoxazoles, obtainable by a process comprising the stepsof

-   (i) mixing (a) one or more aromatic diamino dihydroxy compounds    with (b) one or more aromatic carboxylic acids or esters thereof    comprising at least two acid groups per carboxylic acid monomer,    -   or    -   mixing one or more aromatic and/or heteroaromatic amino hydroxy        carboxylic acids    -   in polyphosphoric acid to form a solution and/or dispersion,-   (ii) heating the mixture from step (i), preferably under inert gas,    to temperatures in the range from 120° C. up to 300° C. to form the    polybenzoxazole polymer,-   (iii) applying a layer using the mixture from step ii) to a support    or to an electrode,-   (iv) at least partially hydrolyzing the polyphosphoric acid present    in the layer from step (iii) by contacting with water and/or aqueous    media,-   (v) detaching the self-supporting membrane formed from the support.

A BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 illustrates the compression resistance of the inventive membraneand of the comparative membrane.

A DETAILED DESCRIPTION OF THE INVENTION

The aromatic diamino dihydroxy compounds used in accordance with theinvention are preferably 3,3′-dihydroxy-4,4′-diaminobiphenyl,3,3′-dihydroxy-4,4′-diaminodiphenyl sulfone,4,6-diamino-1,3-dihydroxybenzene (DABDO) and salts thereof, especiallythe mono- and/or dihydrochloride derivatives thereof.

The aromatic carboxylic acids used in accordance with the invention aredicarboxylic acids, or else dicarboxylic acids in combination withtricarboxylic acids and/or tetracarboxylic acids. Instead of thearomatic carboxylic acids, it is also possible to use the esters,anhydrides or acid chlorides thereof. Among the aromatic dicarboxylicacids, preference is given to those in which the acid groups are in thepara position on the aromatic ring.

The term “aromatic carboxylic acids” likewise also comprisesheteroaromatic carboxylic acids. The aromatic dicarboxylic acids arepreferably isophthalic acid, terephthalic acid, phthalic acid,5-hydroxyisophthalic acid, 4-hydroxyisophthalic acid,2-hydroxyterephthalic acid, 5-aminoisophthalic acid,5-N,N-dimethylaminoisophthalic acid, 5-N,N-diethylaminoisophthalic acid,2,5-dihydroxyterephthalic acid, 2,6-dihydroxyisophthalic acid,4,6-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid,2,4-dihydroxyphthalic acid, 3,4-dihydroxyphthalic acid, 3-fluorophthalicacid, 5-fluoroisophthalic acid, 2-fluoroterephthalic acid,tetrafluorophthalic acid, tetrafluoroisophthalic acid,tetrafluoroterephthalic acid, 1,4-naphthalenedicarboxylic acid,1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid,2,7-naphthalenedicarboxylic acid, diphenic acid,1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, diphenyl ether4,4′-dicarboxylic acid, benzophenone-4,4′-dicarboxylic acid, diphenylsulfone 4,4′-dicarboxylic acid, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid,2,2-bis(4-carboxyphenyl)hexafluoropropane, 4,4′-stilbenedicarboxylicacid and 4-carboxycinnamic acid, or the C1-C20-alkyl esters orC5-C12-aryl esters thereof or acid anhydrides thereof or acid chloridesthereof.

The aromatic tricarboxylic acids or the C1-C20-alkyl esters orC5-C12-aryl esters thereof or acid anhydrides thereof or acid chloridesthereof are preferably 1,3,5-benzenetricarboxylic acid (trimesic acid),1,2,4-benzenetricarboxylic acid (trimellitic acid),(2-carboxyphenyl)iminodiacetic acid, 3,5,3′-biphenyltricarboxylic acid,3,5,4′-biphenyltricarboxylic acid.

The aromatic tetracarboxylic acids or the C1-C20-alkyl esters orC5-C12-aryl esters thereof or acid anhydrides thereof or acid chloridesthereof are preferably 3,5,3′,5′-biphenyltetracarboxylic acid,1,2,4,5-benzenetetracarboxylic acid, benzophenonetetracarboxylic acid,3,3′,4,4′-biphenyltetracarboxylic acid,2,2′,3,3′-biphenyltetracarboxylic acid,1,2,5,6-naphthalenetetracarboxylic acid,1,4,5,8-naphthalenetetracarboxylic acid.

The heteroaromatic carboxylic acids used in accordance with theinvention are heteroaromatic dicarboxylic acids, heteroaromatictricarboxylic acids and heteroaromatic tetracarboxylic acids, or theesters thereof or anhydrides thereof. Heteroaromatic carboxylic acidsare understood to mean aromatic systems which comprise at least onenitrogen, oxygen, sulfur or phosphorus atom in the aromatic system. Theyare preferably pyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylicacid, pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid,4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic acid,2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid,2,4,6-pyridinetricarboxylic acid, benzimidazole-5,6-dicarboxylic acid,and the C1-C20-alkyl esters or C5-C12-aryl esters thereof, or acidanhydrides thereof or acid chlorides thereof.

The content of tricarboxylic acid or tetracarboxylic acids (based on thedicarboxylic acid used) is between 0 and 30 mol %, preferably 0.1 and 20mol %, especially 0.5 and 10 mol %.

The aromatic and/or heteroaromatic amino hydroxy carboxylic acids usedin accordance with the invention are preferably 3-amino-4-hydroxybenzoicacid and 4-amino-3-hydroxybenzoic acid.

Preference is given to using, in step A), mixtures of at least 2different aromatic carboxylic acids. Particular preference is given tousing mixtures which comprise, as well as aromatic carboxylic acids,also heteroaromatic carboxylic acids. The mixing ratio of aromaticcarboxylic acids to heteroaromatic carboxylic acids is between 1:99 and99:1, preferably 1:50 to 50:1, especially 1:10 to 10:1.

These mixtures are especially mixtures of N-heteroaromatic dicarboxylicacids and aromatic dicarboxylic acids. Nonlimiting examples thereof areisophthalic acid, terephthalic acid, phthalic acid,2,5-dihydroxyterephthalic acid, 2,6-dihydroxyisophthalic acid,4,6-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid,2,4-dihydroxyphthalic acid, 3,4-dihydroxyphthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, diphenylether 4,4′-dicarboxylic acid, benzophenone-4,4′-dicarboxylic acid,diphenyl sulfone 4,4′-dicarboxylic acid, biphenyl-4,4′-dicarboxylicacid, 4-trifluoromethylphthalic acid, pyridine-2,5-dicarboxylic acid,pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid,pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid,3,5-pyrazoledicarboxylic acid, 2,6-pyrimidinedicarboxylic acid,2,5-pyrazinedicarboxylic acid.

The polyphosphoric acid used in step A) comprises commercialpolyphosphoric acids as obtainable, for example, from Riedel-de Haen.The polyphosphoric acids H_(n+2)P_(n)O_(3n+1) (n>1) typically have acontent, calculated as P₂O₅ (by acidimetric means) of at least 79.8%,which corresponds to a concentration of min. 110% H₃PO₄. Instead of asolution of the monomers, it is also possible to produce adispersion/suspension.

The mixture obtained in step A) has a weight ratio of polyphosphoricacid to the sum of all monomers of 1:10 000 to 10 000:1, preferably1:1000 to 1000:1, especially 1:100 to 100:1.

“Polyoxazole” is understood to mean polymers which have at least oneoxygen heteroatom and at least one nitrogen heteroatom in the aromaticsystem. The aromatic system may be mono- or polycyclic and alsocomprises fused aromatic ring systems. Particular preference is given toaromatic systems in which one aromatic ring has at least one oxygenheteroatom and at least one nitrogen heteroatom.

The aromatic ring is preferably a five- or six-membered ring having onenitrogen atom and one oxygen atom, which may be fused to another ring,especially another aromatic ring.

A polymer having “high thermal stability” in the context of the presentinvention is one which can be operated for a prolonged period as apolymeric electrolyte in a fuel cell at temperatures above 120° C. “Fora prolonged period” means that an inventive membrane can be operated forat least 100 hours, preferably at least 500 hours, at at least 80° C.,preferably at least 120° C., more preferably at least 160° C., withoutany decrease in the performance, which can be measured by the methoddescribed in WO 01/18894 A2, by more than 50%, based on the startingperformance.

A particularly preferred group of polyoxazole polymers comprises thosewhich comprise repeat oxazole units of the general formula (I) and/or(II) and/or (III) and/or (IV) and/or (V) and/or (VI) and/or (VII)

in which

-   Ar is the same or different and is a tetravalent aromatic or    heteroaromatic group, which may be mono- or polycyclic,-   Ar¹ is the same or different and is a divalent aromatic or    heteroaromatic group, which may be mono- or polycyclic,-   Ar² is the same or different and is a di- or trivalent aromatic or    heteroaromatic group, which may be mono- or polycyclic,-   Ar³ is the same or different and is a trivalent aromatic or    heteroaromatic group, which may be mono- or polycyclic,-   Ar⁴ is the same or different and is a trivalent aromatic or    heteroaromatic group, which may be mono- or polycyclic,-   Ar⁵ is the same or different and is a tetravalent aromatic or    heteroaromatic group, which may be mono- or polycyclic,-   Ar⁶ is the same or different and is a divalent aromatic or    heteroaromatic group, which may be mono- or polycyclic,-   Ar⁷ is the same or different and is a divalent aromatic or    heteroaromatic group, which may be mono- or polycyclic,-   Ar⁸ is the same or different and is a divalent aromatic or    heteroaromatic group, which may be mono- or polycyclic,-   Ar⁹ is the same or different and is a divalent aromatic or    heteroaromatic group, which may be mono- or polycyclic,-   X is the same or different and is oxygen,-   n is an integer greater than or equal to 10, preferably greater than    or equal to 100.

Preferred aromatic or heteroaromatic groups derive from benzene,naphthalene, biphenyl, diphenyl ether, diphenylmethane,diphenyldimethylmethane, bisphenone, diphenylsulfone, quinoline,pyridine, bipyridine, pyridazine, pyrimidine, pyrazine, triazine,tetrazine, pyrrole, pyrazole, anthracene, benzopyrrole, benzotriazole,benzoxathiadiazole, benzoxadiazole, benzopyridine, benzopyrazine,benzopyrazidine, benzopyrimidine, benzopyrazine, benzotriazine,indolizine, quinolizine, pyridopyridine, imidazopyrimidine,pyrazinopyrimidine, carbazole, aciridine, phenazine, benzoquinoline,phenoxazine, phenothiazine, acridizine, benzopteridine, phenanthrolineand phenanthrene, which may optionally also be substituted.

The substitution pattern of Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹ is as desired;in the case of phenylene, for example, Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹ maybe ortho-, meta- and para-phenylene. Particularly preferred groupsderive from benzene and biphenylene, which may optionally also besubstituted.

Preferred alkyl groups are short-chain alkyl groups having 1 to 4 carbonatoms, for example methyl, ethyl, n- or i-propyl and t-butyl groups.

Preferred aromatic groups are phenyl or naphthyl groups. The alkylgroups and the aromatic groups may be substituted.

Preferred substituents are halogen atoms, for example fluorine, aminogroups, hydroxy groups or short-chain alkyl groups, for example methylor ethyl groups.

Preference is given to polyoxazoles having repeat units of the formula(I) in which the X radicals are the same within one repeat unit.

The polyoxazoles may in principle also have different repeat units whichdiffer, for example, in their X radical. However, it preferably has onlyidentical X radicals in one repeat unit.

Further preferred polyoxazole polymers are polybenzoxazoles.

In a further embodiment of the present invention, the polymer comprisingrepeat azole units is a copolymer or a blend which comprises at leasttwo units of the formulae (I) to (VII) which differ from one another.The polymers may be in the form of block copolymers (diblock, triblock),random copolymers, periodic copolymers and/or alternating polymers.

In a particularly preferred embodiment of the present invention, thepolymer comprising repeat oxazole units is a polyoxazole which comprisesonly units of the formula (I) and/or (II).

The number of repeat oxazole units in the polymer is preferably aninteger greater than or equal to 10. Particularly preferred polymerscomprise at least 100 repeat oxazole units.

In the context of the present invention, preference is given to polymerscomprising repeat units of the following formulae:

where n in each case is an integer greater than or equal to 10,preferably greater than or equal to 100.

The polyoxazole used, but especially the polybenzoxazoles, are notablefor a high molecular weight. Measured as the intrinsic viscosity, it isat least 0.2 dl/g, preferably 0.8 to 10 dl/g, especially 1 to 10 dl/g.

The preparation of polyoxazole is known in principle, the parentmonomers being converted to a prepolymer in the melt. The resultingprepolymer solidifies in the reactor and is then mechanicallycomminuted. The pulverulent prepolymer is typically finally polymerizedin a solid phase polymerization at temperatures of up to 400° C.

It is additionally known, especially in the case of preparation oflaboratory amounts, that the parent monomers can be condensed inpolyphosphoric acid and then precipitated by introduction into water andwashed to neutrality.

The aforementioned polyoxazole polymers can be used individually or as amixture (blend). Preference is given here especially to blends whichcomprise polyazoles as described in WO 02/088219, WO 02/081547, WO03/022412, and/or polysulfones. The preferred blend components arepolyether sulfone, polyether ketone, and polymers modified with sulfogroups, as described in patent application EP-A-1337319 and US2004/075172. The use of blends can improve the mechanical properties andreduce the material costs.

When the inventive polyoxazoles, more particularly the polybenzoxazoles,are to be used as a blend, they are admixed with the polymers describedhereinafter, but especially with polysulfones and/or polyether sulfone,or added as early as in the course of production of the polyoxazole, forexample in step i) (reactor blend).

The preferred polysulfones include especially polysulfone with aromaticand/or heteroaromatic groups in the main chain. In a particular aspectof the present invention, preferred polysulfones and polyether sulfoneshave a melt volume flow rate MVR 300/21.6 less than or equal to 40cm³/10 min, especially less than or equal to 30 cm³/10 min and morepreferably less than or equal to 20 cm³/10 min, measured to ISO 1133.Preference is given here to polysulfones having a Vicat softeningtemperature VST/A/50 of 180° C. to 230° C. In another preferredembodiment of the present invention, the number-average molecular weightof the polysulfones is greater than 30 000 g/mol.

The polymers based on polysulfone include especially polymers which haverepeat units with linking sulfone groups according to the generalformulae A, B, C, D, E, F and/or G:

in which the R radicals are the same or different and are eachindependently an aromatic or heteroaromatic group, these radicals havingbeen elucidated in detail above. These include especially 1,2-phenylene,1,3-phenylene, 1,4-phenylene, 4,4′-biphenyl, pyridine, quinoline,naphthalene, phenanthrene.

The polysulfones preferred in the context of the present inventioninclude homo- and copolymers, for example random copolymers.Particularly preferred polysulfones comprise repeat units of theformulae H to N:

The above-described polysulfones can be obtained commercially under thetrade names ®Victrex 200 P, ®Victrex 720 P, ®Ultrason E, ®Ultrason S,®Mindel, ®Radel A, ®Radel R, ®Victrex HTA, ®Astrel and ®Udel.

In addition, particular preference is given to polyether ketones,polyether ketone ketones, polyether ether ketones, polyether etherketone ketones and polyaryl ketones. These high-performance polymers areknown per se and can be obtained commercially under the trade namesVictrex® PEEK™, ®Hostatec, ®Kadel.

Further suitable blend materials are the polyazoles described in WO02/088219, WO 02/081547, WO 03/022412. These can be obtained by addingthe corresponding monomers to the mixture in step i) in polyphosphoricacid. For this purpose, the monomers for the polyazole blend materialare used in amounts of up to 100 mol %, based on the monomers for thepolyoxazole, such that not more than 50 mol % of polyazole in the blendpresent and the remaining 50 mol % is polyoxazole. The lower limit isnot subject to any restriction, but preference is given to 5 to 10 mol %of monomers of the polyazole based on the monomers for the polyoxazole.Below these amounts, the contributions of the polyazole blend materialare very limited. Addition of polyazoles allows the excellent useproperties of polyoxazoles to be improved further.

The mixture is heated in step ii) within the temperature range from 120to 300° C. It is advantageous in this context to increase thetemperature stepwise, preferably in intervals of 20-30° C. The durationof the heating is typically between 2 and 100 hours, preferably between5 and 80 hours.

The layer formation in step iii) is effected by means of measures knownper se (casting, spraying, knife-coating), which are known from theprior art for polymer film production. Suitable supports are allsupports which can be described as inert under the conditions. To adjustthe viscosity, phosphoric acid (conc. phosphoric acid, 85%) canoptionally be added to the solution. The viscosity can thus be adjustedto the desired value and the formation of the membrane can befacilitated.

The layer obtained in step iii) has a thickness between 20 and 4000 μm,preferably between 30 and 3500 μm, especially between 50 and 3000 μm.

In one variant of the process, heating in step ii) may also followformation of the layer in step iii). This thin-layer polymerization isin principle effected within the temperature range from 120 to 300° C.,though higher temperatures can also be applied briefly. Instead of apurely thermal heating operation, it is also possible to use high-energyelectromagnetic radiation, for example IR and/or NIR radiation,microwaves and the like. In addition, initial heating, for example atthe lower end of the temperature range, can also be followed by furtherheating in the form of said layer.

In one variant of the present invention, the membrane can also be formeddirectly on the electrode, instead of on a support. The at least partialhydrolysis in step iv) can correspondingly be shortened as a result,since there is no longer any need for the membrane to beself-supporting. Such a membrane also forms part of the subject matterof the present invention.

The at least partial hydrolysis of the polyphosphoric acid still presentin step iv) is effected by contacting the membrane present on thesupport with water and/or an aqueous medium.

The hydrolysis is effected preferably at temperatures above 0° C. andbelow 200° C., preferably at temperatures between 10° C. and 120° C.,especially between room temperature (20° C.) and 90° C. A suitableaqueous medium is water and/or water vapor and/or water-containingphosphoric acid of up to 85%. The hydrolysis is effected preferablyunder standard pressure, but can also be effected under the action ofpressure. It is essential that the treatment proceeds in the presence ofsufficient humidity, as a result of which the polyphosphoric acidpresent is at least partially hydrolyzed. This forms substancesincluding low molecular weight polyphosphoric acid and/or phosphoricacid, which contribute to the consolidation of the membrane. When theambient air has sufficient air humidity, for example relative humiditymin. 30%, the hydrolysis can also be effected by the ambient air.

As well as partial hydrolysis, complete hydrolysis of the polyphosphoricacid present is also possible.

The partial hydrolysis of the polyphosphoric acid in step iv) leads toconsolidation of the membrane and to a decrease in the layer thickness,and formation of a membrane which has a thickness between 15 and 3000μm, preferably between 20 and 2000 μm, especially between 20 and 1500μm, and which is self-supporting.

The intra- and intermolecular structures present in the polyphosphoricacid layer in step iii) (interpenetrating networks, IPNs) lead, in stepiv), to ordered membrane formation which is found to be responsible forthe exceptional properties of the membrane formed.

The at least partial hydrolysis in step iv) causes a sol-gel transitionand leads to a rubber-like membrane in which the polybenzoxazole actslike a superabsorbent for the polyphosphoric acid/phosphoric acid. Theinventive membranes have a high content of phosphoric acid and are notcomparable with subsequently doped membranes.

The at least partial hydrolysis (step iv) can also be effected inclimate-controlled chambers in which the hydrolysis can be controlledunder defined action of moisture. In this case, the humidity can beadjusted in a controlled manner via the temperature or saturation of thecontact environment, for example gases such as air, nitrogen, carbondioxide or other suitable gases, or water vapor. The treatment timedepends on the parameters selected above.

In addition, the treatment time depends on the thickness of themembrane.

In general, the treatment time is between a few seconds and minutes, forexample under the action of superheated steam, or up to whole days, forexample under air at room temperature and low relative air humidity. Thetreatment time is preferably between 10 seconds and 300 hours,especially 1 minute to 200 hours.

When the at least partial hydrolysis is performed at room temperature(20° C.) with ambient air of relative air humidity 40-80%, the treatmenttime is between 1 and 200 hours.

The membrane obtained in step iv) can be configured so as to beself-supporting, i.e. it can be detached without damage from the supportand then optionally processed further directly.

It is possible to adjust the concentration of phosphoric acid and hencethe conductivity of the inventive polymer membrane via the degree ofhydrolysis, i.e. the time, temperature and ambient humidity.

The at least partial hydrolysis can also be effected in an aqueousliquid, in which case this liquid may also comprise suspended and/ordispersed constituents. The viscosity of the hydrolysis liquid may bewithin wide ranges, and the viscosity can be adjusted by adding solventsor increasing the temperature. The dynamic viscosity is preferably inthe range from 0.1 to 10 000 mPa*s, especially 0.2 to 2000 mPa*s, thesevalues being measurable, for example, to DIN 53015.

The at least partial hydrolysis in step iv) can be effected by any knownmethod. For example, the membrane obtained in step iii) can be immersedinto a liquid bath. In addition, the hydrolysis liquid can be sprayedonto the membrane. The hydrolysis liquid can also be poured over themembrane. The latter methods have the advantage that the concentrationof acid in the hydrolysis liquid remains constant during the hydrolysis.However, the first process is frequently less expensive to execute.

The hydrolysis liquid comprises aqueous mixtures of oxygen acids ofphosphorus and/or sulfur, especially phosphinic acid, phosphonic acid,phosphoric acid, hypodiphosphonic acid, hypodiphosphoric acid,oligophosphoric acids, sulfurous acid, disulfurous acid and/or sulfuricacid. These acids can be used individually or as a mixture.

The hydrolysis liquid comprises water, though the concentration of thewater is generally not particularly critical. In a particular aspect ofthe present invention, the hydrolysis liquid comprises 5 to 80% byweight, preferably 8 to 70% by weight and more preferably 10 to 50% byweight of water. The amount of water present formally in the oxygenacids is not included in the water content of the hydrolysis liquid.

Among the aforementioned acids, phosphoric acid and/or sulfuric acid areparticularly preferred, these acids comprising especially 5 to 70% byweight, preferably 10 to 60% by weight and more preferably 15 to 50% byweight of water.

According to the invention, the concentration of the phosphoric acid isreported as moles of acid per mole of repeat unit of the polymer. In thecontext of the present invention, a concentration (moles of phosphoricacid based on one repeat unit of the formula C18H₁₀N₂O₂, i.e.polybenzoxazole) is between 10 and 50, preferably between 13 and 40 andespecially between 15 and 35 mol.

Such high levels of doping (concentrations) are not obtainable bysubsequent doping of polymer films.

The inventive membranes comprise preferably between 2 and 15% by weightof polyoxazoles and between 40 and 70% by weight of phosphoric acid, theremaining amount being water. Particular preference is given topolyoxazole contents of 5 to 10% by weight and proportions of phosphoricacid of 50 to 60% by weight, the remaining amount being water.

After the hydrolysis in step iv) or after the detachment in step v), themembrane can also be surface treated by the action of heat in thepresence of atmospheric oxygen. This curing of the membrane surfaceadditionally improves the properties of the membrane.

This treatment can also be effected by action of IR or NIR (IR=infrared,i.e. light with a wavelength of more than 700 nm; NIR=near IR, i.e.light with a wavelength in the range from approx. 700 to 2000 nm or anenergy in the range from approx. 0.6 to 1.75 eV). A further method isirradiation with β rays. The radiation dose here is between 5 and 200kGy.

The inventive polymer membrane has improved material properties over thepolymer membranes known to date.

The inventive membranes have a good proton conductivity. The protonconductivity at temperatures of 160° C. is at least 0.1 S/cm, preferablyat least 0.105 S/cm. The proton conductivity is determined withoutadditional moistening of the gases required.

For additional improvement of the performance properties, fillers,especially proton-conducting fillers, and additional acids canadditionally be added to the membrane. The addition can either beeffected in step i) or may follow the polymerization.

Nonlimiting examples of proton-conducting fillers are

-   sulfates such as: CsHSO₄, Fe(SO₄)₂, (NH₄)₃H(SO₄)₂, LiHSO₄, NaHSO₄,    KHSO₄, RbSO₄, LiN₂H₅SO₄, NH₄HSO₄,-   phosphates such as Zr₃(PO₄)₄, Zr(HPO₄)₂, HZr₂(PO₄)₃, UO₂PO₄.3H₂O,    H₈UO₂PO₄, Ce(HPO₄)₂, Ti(HPO₄)₂, KH₂PO₄, NaH₂PO₄, LiH₂PO₄, NH₄H₂PO₄,    CsH₂PO₄, CaHPO₄, MgHPO₄, HSbP₂O₈, HSb₃P₂O₁₄, H₅Sb₅P₂O₂₀,-   polyacid such as H₃PW₁₂O₄₀.nH₂O (n=21-29), H₃SiW₁₂O₄₀.nH₂O    (n=21-29), H_(x)WO₃, HSbWO₆, H₃PMo₁₂O₄₀, H₂Sb₄O₁₁, HTaWO₆, HNbO₃,    HTiNbO₅, HTiTaO₅, HSbTeO₆, H₅Ti₄O₉, HSbO₃, H₂MoO₄-   selenites and arsenides such as (NH₄)₃H(SeO₄)₂, UO₂AsO₄,    (NH₄)₃H(SeO₄)₂, KH₂AsO₄, Cs₃H(SeO₄)₂, Rb₃H(SeO₄)₂,-   phosphides such as ZrP, TiP, HfP-   oxides such as Al₂O₃, Sb₂O₅, ThO₂, SnO₂, ZrO₂, MoO₃-   silicates such as zeolites, zeolites (NH₄+), sheet silicates,    framework silicates, H-natrolites, H-mordenites, NH₄-analcines,    NH₄-sodalites, NH₄-gallates, H-montmorillonites,-   acids such as HClO₄, SbF₅-   fillers such as carbides, especially SiC, Si₃N₄, fibers, especially    glass fibers, glass powders and/or polymer fibers, preferably based    on polyazoles.

This membrane may also further comprise perfluorinated sulfonic acidadditives (0.1-20% by weight, preferably 0.2-15% by weight, verypreferably 0.2-10% by weight). These additives lead to improvedperformance, in the vicinity of the cathode to an increase in the oxygensolubility and oxygen diffusion, and to a reduction in the adsorption ofphosphoric acid and phosphate to platinum. (Electrolyte additives forphosphoric acid fuel cells. Gang, Xiao; Hjuler, H. A.; Olsen, C.; Berg,R. W.; Bjerrum, N.J., Chem. Dep. A, Tech. Univ. Denmark, Lyngby, Den.,J. Electrochem. Soc. (1993), 140(4), 896-902 and Perfluorosulfonimide asan additive in phosphoric acid fuel cell. Razaq, M.; Razaq, A.; Yeager,E.; DesMarteau, Darryl D.; Singh, S., Case Cent. Electrochem. Sci., CaseWest. Reserve Univ., Cleveland, Ohio, USA., J. Electrochem. Soc. (1989),136(2), 385-90.).

Nonlimiting examples of persulfonated additives are:trifluoromethanesulfonic acid, potassium trifluoromethanesulfonate,sodium trifluoromethanesulfonate, lithium trifluoromethanesulfonate,ammonium trifluoromethanesulfonate, potassium perfluorohexanesulfonate,sodium perfluorohexanesulfonate, lithium perfluorohexanesulfonate,ammonium perfluorohexanesulfonate, perfluorohexanesulfonic acid,potassium nonafluorobutanesulfonate, sodium nonafluorobutanesulfonate,lithium nonafluorobutanesulfonate, ammonium nonafluorobutanesulfonate,cesium nonafluorobutanesulfonate, triethylammoniumperfluorohexanesulfonate, perfluorosulfoimides and Nafion.

The membrane may also further comprise as additives which scavenge(primary antioxidants) or destroy (secondary antioxidants) the peroxideradicals generated in oxygen reduction in the course of operation andthus, as described in JP2001118591 A2, improve lifetime and stability ofthe membrane and membrane electrode assembly. The way in which suchadditives work and their molecular structures are described in F.Gugumus in Plastics Additives, Hanser Verlag, 1990; N. S. Allen, M. EdgeFundamentals of Polymer Degradation and Stability, Elsevier, 1992; or H.Zweifel, Stabilization of Polymeric Materials, Springer, 1998.

Nonlimiting examples of such additives are:bis(trifluoromethyl)nitroxide, 2,2-diphenyl-1-picrinylhydrazyl, phenols,alkylphenols, sterically hindered alkylphenols, for example Irganox,aromatic amines, sterically hindered amines, for example Chimassorb;sterically hindered hydroxylamines, sterically hindered alkylamines,sterically hindered hydroxylamines, sterically hindered hydroxylamineethers, phosphites, for example Irgafos, nitrosobenzene,methyl-2-nitrosopropane, benzophenone, benzaldehyde tert-butyl nitron,cysteamine, melanines, lead oxides, manganese oxides, nickel oxides,cobalt oxides.

The inventive polymer membrane has additional improved materialproperties compared to the polymer membranes which are based onpolyazoles and are known to date. For instance, the inventive membranesbased on polyoxazoles exhibit improved compression resistance. Theimproved compression resistance results in an improved long-termstability with equal or virtually equal electrochemical performance.

The inventive polyazole membranes have, at operating temperatures of themembrane electrode assemblies of more than 100° C., preferably of180-200° C., a compression resistance improved by a factor of 2 comparedto polybenzimidazole membranes. This is found in an external test cellknown to those skilled in the art, by the measurement of the decrease inthickness of a membrane sample within a given time window under theaction of a profiled test body at the relevant operating temperatures.

In one modification, the inventive membrane based on polyoxazoles canalso be produced by a process comprising the steps of

-   1) converting a mixture of (a) one or more aromatic diamino    dihydroxy compounds and (b) one or more aromatic carboxylic acids or    esters thereof comprising at least two acid groups per carboxylic    acid monomer,    -   or    -   a mixture of one or more aromatic and/or heteroaromatic amino        hydroxy carboxylic acids    -   in the melt at temperatures of up to 350° C., preferably up to        300° C.,-   2) dissolving the solid prepolymer obtained in step 1) in    polyphosphoric acid,-   3) heating the solution obtainable in step 2), preferably under    inert gas, to temperatures in the range from 120° C. up to 300° C.    to form the polybenzoxazole polymer,-   4) forming a membrane using the solution of the polybenzoxazole    polymer according to step 3) on a support,-   5) at least partially hydrolyzing the polyphosphoric acid present in    the layer from step (iii) by contacting with water and/or aqueous    media,-   6) detaching the self-supporting membrane formed from the support.

The process steps described in points 1) to 5) have already beenexplained in detail above for steps i) to v), and reference is madethereto, especially with regard to preferred embodiments.

The possible fields of use of the inventive doped polymer membranesinclude use in fuel cells, in electrolysis, in capacitors and in batterysystems. Due to their profile of properties, the doped polymer membranesare preferably used in fuel cells.

The present invention also relates to a membrane electrode assemblycomprising at least one inventive polymer membrane. For furtherinformation about membrane electrode assemblies, reference is made tothe technical literature, especially to the U.S. Pat. No. 4,191,618,U.S. Pat. No. 4,212,714 and U.S. Pat. No. 4,333,805. The disclosure inthe aforementioned references [U.S. Pat. No. 4,191,618, U.S. Pat. No.4,212,714 and U.S. Pat. No. 4,333,805] with regard to the constructionand the production of membrane electrode assemblies, and the electrodes,gas diffusion layers and catalysts to be selected, also forms part ofthe description.

The inventive membrane electrode assembly has two gas diffusion layersseparated by the polymer electrolyte membrane. Typically used for thispurpose are flat, electrically conductive and acid-resistant structures.These include, for example, graphite fiber papers, carbon fiber papers,graphite mesh and/or papers which have been rendered conductive byaddition of carbon black. These layers achieve fine distribution of thegas and/or liquid streams.

This layer generally has a thickness in the range from 80 μm to 2000 μm,especially 100 μm to 1000 μm and more preferably 150 μm to 500 μm.

In a particular embodiment, at least one of the gas diffusion layers mayconsist of a compressible material. In the context of the presentinvention, a compressible material is characterized by the property thatthe gas diffusion layer can be compressed to half, especially to onethird, of its original thickness without loosing its integrity.

This property is generally possessed by gas diffusion layers composed ofgraphite mesh and/or paper which has been rendered conductive byaddition of carbon black.

As well as the two gas diffusion layers separated by the polymerelectrolyte membrane, the inventive membrane electrode assembly also hasa catalyst layer on each side of the membrane. The catalyst layer may bepresent on both sides of the membrane or at the interface of the gasdiffusion layer to the membrane.

The catalyst layer(s) comprise(s) catalytically active substances. Theseinclude noble metals of the platinum group, i.e. Pt, Pd, Ir, Rh, Os, Ru,or else the noble metals Au and Ag. In addition, it is also possible touse alloys of all aforementioned metals. In addition, at least onecatalyst layer may comprise alloys of the platinum group elements withbase metals, for example Fe, Co, Ni, Cr, Mn, Zr, Ti, Ga, V etc. Inaddition, it is also possible to use the oxides of the aforementionednoble metals and/or base metals.

The catalytically active particles which comprise the aforementionedsubstances can be used in the form of metal powders, known as noblemetal blacks, especially platinum and/or platinum alloys. Such particlesgenerally have a size in the range from 5 nm to 200 nm, preferably inthe range from 7 nm to 100 nm.

In addition, the metals can also be used on a support material. Thissupport preferably comprises carbon, which can be used especially in theform of carbon black, graphite or graphitized carbon black. In addition,it is also possible to use electrically conductive metal oxides, forexample SnO_(x), TiO_(x), or phosphates, for example FePO_(x), NbPO_(x),Zr_(y)(PO_(x))_(z) as support material. In these formulae, the indicesx, y and z denote the oxygen or metal content of the individualcompounds, which may be within a known range, since the transitionmetals can assume different oxidation states.

The content of these supported metal particles, based on the totalweight of the metal-support compound, is generally in the range from 1to 80% by weight, preferably 5 to 60% by weight and more preferably 10to 50% by weight, without any intention that this should impose arestriction. The particle size of the support, especially the size ofthe carbon particles, is preferably in the range from 20 to 1000 nm,especially 30 to 100 nm. The size of the metal particles present thereonis preferably in the range from 1 to 20 nm, especially 1 to 10 nm andmore preferably 2 to 6 nm.

The sizes of the different particles are averages and can be determinedby means of transmission electron microscopy or x-ray powderdiffractometry.

The catalytically active particles detailed above can generally beobtained commercially.

In addition, the catalytically active layer may comprise customaryadditives. These include fluoropolymers, for examplepolytetrafluoroethylene (PTFE), proton-conducting ionomers andsurface-active substances.

In a particular embodiment of the present invention, the weight ratio offluoropolymer to catalyst material comprising at least one noble metaland optionally one or more support materials is greater than 0.1, thisratio preferably being in the range from 0.2 to 0.6.

In a particular embodiment of the present invention, the catalyst layerhas a thickness in the range from 1 to 1000 μm, especially from 5 to 500μm, preferably from 10 to 300 μm. This value is an average which can bedetermined by measuring the layer thickness in the cross section ofimages obtainable with a scanning electron microscope (SEM).

In a particular embodiment of the present invention, the noble metalcontent of the catalyst layer is 0.1 to 10.0 mg/cm², preferably 0.3 to6.0 mg/cm² and more preferably 0.3 to 3.0 mg/cm². These values can bedetermined by elemental analysis of a flat sample.

The production of inventive membrane electrode assemblies is obvious tothe person skilled in the art. In general, the different constituents ofthe membrane electrode assembly are placed one on top of another andbonded to one another by pressure and temperature. In general,lamination is effected at a temperature in the range from 10 to 300° C.,especially 20 to 200° C., and with a pressure in the range from 1 to1000 bar, especially from 3 to 300 bar.

The finished membrane electrode assembly (MEA) is ready for operationafter cooling and can be used in a fuel cell.

The inventive membrane electrode assembly (MEA) is suitable foroperation at temperatures above 160° C. and enables gaseous and/orliquid fuels, for example hydrogen-comprising gases, which are prepared,for example, in an upstream reforming step from hydrocarbons. Theoxidant used may, for example, be oxygen or air.

A further advantage of the inventive membrane electrode assembly is thatthey have a high tolerance to carbon monoxide in operation above 120° C.even with pure platinum catalysts, i.e. without a further alloyconstituent. At temperatures of 160° C. for example, more than 1% CO maybe present in the fuel gas without this leading to any noticeablereduction in the performance of the fuel cell.

The inventive membrane electrode assembly can be operated in fuel cellswithout any need to moisten the fuel gases and the oxidants in spite ofthe high operating temperatures possible. The fuel cell neverthelessworks stably and the membrane does not lose its conductivity. Thissimplifies the overall fuel cell system and brings additional costsavings since the control of the water circuit (cooling) is simplified.

The inventive membrane electrode assembly can be cooled without anydifficulty to room temperature and below and can then put back intooperation, without losing performance.

Furthermore, the inventive MEAs can be produced in an inexpensive andsimple manner.

The inventive proton-conducting polymer membrane based on polyoxazolesis notable for a considerable improvement in compression resistance. Theinventive membranes exhibit a much lower decrease in thickness at 200°C. For instance, a membrane based on polyoxazoles, in the test methoddescribed, still has a residual thickness of min. 50% after 10 minutesat 200° C., while a comparable membrane based on polyazoles(polybenzimidazole) has only a residual thickness of about 40%.

Preferably, an inventive membrane based on polyoxazole, in the testmethod described, has a residual thickness of min. 40% after 20 minutesat 200° C., while a comparable membrane based on polyazoles(polybenzimidazole) has only a residual thickness of less than 30%.

More preferably, an inventive membrane based on polyoxazole, in the testmethod described, has a residual thickness of min. 35% after 60 minutesat 200° C., while a comparable membrane based on polyazoles(polybenzimidazole) has only a residual thickness of less than 25%.

General Test Methods:

Test Method for IEC

The conductivity of the membrane depends significantly on the content ofacid groups expressed by what is called the ion exchange capacity (IEC).To measure the ion exchange capacity, a sample with a diameter of 3 cmis punched out and introduced into a beaker filled with 100 ml of water.The acid released is titrated with 0.1 M NaOH. Subsequently, the sampleis withdrawn, excess water is dabbed off and the sample is dried at 160°C. over 4 h. Then the dry weight, m₀, is determined gravimetrically withan accuracy of 0.1 mg. The ion exchange capacity is then calculated fromthe consumption of the 0.1 M NaOH up to the first titration end point,V₁ in ml, and the dry weight, m₀ in mg, by the following formula:IEC=V ₁*300/m ₀

Test Method for Specific Conductivity

The specific conductivity is measured by means of impedance spectroscopyin a 4-pole arrangement in potentiostatic mode using platinum electrodes(wire, diameter 0.25 mm). The distance between the current-collectingelectrodes is 2 cm. The resulting spectrum is evaluated with a simplemodel consisting of a parallel arrangement of an ohmic resistance and acapacitor. The sample cross section of the phosphoric acid-dopedmembrane is measured immediately before the sample mounting. To measurethe temperature dependence, the test cell is brought to the desiredtemperature in an oven and regulated by means of a Pt-100 thermocouplepositioned in the immediate vicinity of the sample. On attainment of thetemperature, the sample is kept at this temperature for 10 minutesbefore the start of the measurement.

Test Method for Compression Resistance

A membrane specimen of diameter 4.91 cm² (d=2.5 cm) is punched out andpositioned on a piece of Kapton film on a hotplate. A metal plunger withthree recesses (depth=1 mm, width 2 mm, length 1.5 cm, separation 5 mmin each case) is positioned by means of guide rails on the membranesample and the compression resistance is assessed by measuring thedecrease in thickness at 200° C. (±10° C.) by means of a Mitutoyo DC IIIfor a period of 120 minutes. The decrease in thickness in [%] iscalculated by [thickness_(after)/thickness_(start)]×100.

The present invention is illustrated in detail hereinafter by an exampleand a comparative example, without any intention that this should imposea restriction.

Example 1

A solution of 4% by weight containing equimolar amounts of 19.76 g of3,3′-dihydroxy-4,4′-diaminobiphenyl and 15.25 g of terephthalic acid inpolyphosphoric acid (116%) is heated to 240° C. within 40 h. Theresulting polybenzoxazole-polyphosphoric acid solution is cooled to atemperature of 100° C. and applied by means of a manual coating bar to asupport in a 450 μm-thick layer and, after cooling, hydrolyzed in 50% byweight phosphoric acid overnight to obtain a self-supportingpolybenzoxazole-phosphoric acid membrane. The membrane properties arelisted in Table 1.

Comparative Example

A solution of 2% by weight containing equimolar amounts of3,3′,4,4′-tetraminobiphenyl and terephthalic acid in polyphosphoric acid(112%) is heated to 280° C. within 100 h. The resultingpolybenzimidazole-polyphosphoric acid solution is cooled to atemperature of 100° C. and applied by means of a manual coating bar to asupport in a 450 μm-thick layer and, after cooling, hydrolyzed in 50% byweight phosphoric acid overnight to obtain a self-supportingpolybenzimidazole-phosphoric acid membrane. The membrane properties arelisted in Table 1.

TABLE 1 Comparison of the properties of reinforced and standard membraneComparative example Example Thickness  390 μm  455 μm Phosphoric acidcontent 54.1% by wt. 52.4% by wt. Solids content  5.6% by wt.  6.5% bywt. Acid concentration 57.3% by wt. 56.0% by wt. Inherent viscosity  5.7dL/g  1.9 dL/g Proton conductivity  100 mS/cm at 160° C.  105 mS/cm at160° C.

The compression resistance of the inventive membrane and of thecomparative membrane are shown in FIG. 1, the measurement having beenperformed as explained above.

The invention claimed is:
 1. A proton-conducting polymer membrane basedon polyoxazoles, obtained by a process comprising the steps of (i)mixing (a) one or more aromatic diamino dihydroxy compounds with (b) oneor more aromatic carboxylic acids or esters thereof comprising at leasttwo acid groups per carboxylic acid monomer, or mixing one or morearomatic and/or heteroaromatic amino hydroxy carboxylic acids inpolyphosphoric acid to form a solution and/or dispersion, (ii) heatingthe mixture from step (i), to temperatures in the range from 120° C. upto 300° C. to form polybenzoxazole polymer, (iii) applying a layer usingthe mixture from step ii) to a support or to an electrode, (iv) at leastpartially hydrolyzing the polyphosphoric acid present in the layer fromstep (iii) by contacting with water and/or aqueous media, (v) detachingthe self-supporting membrane formed from the support and wherein themembrane has a proton conductivity, measured at 160° C. and withoutadditional moistening of the gases required, of at least 0.1 S/cm andthe content of polyoxazoles is between 2 and 15% by weight and thecontent of phosphoric acid between 40 and 70% by weight, and theremaining amount is water.
 2. The membrane according to claim 1, whereinthe aromatic diamino dihydroxy compounds used are3,3′-dihydroxy-4,4′-diaminobiphenyl, 3,3′-dihydroxy-4,4′-diaminodiphenylsulfone, 4,6-diamino-1,3-dihydroxybenzene (DABDO) or their saltsthereof, or mono- and/or dihydrochloride derivatives thereof.
 3. Themembrane according to claim 1, wherein said one or more aromaticcarboxylic acids used are isophthalic acid, terephthalic acid, phthalicacid, 5-hydroxyisophthalic acid, 4-hydroxyisophthalic acid,2-hydroxyterephthalic acid, 5-aminoisophthalic acid,5-N,N-dimethylaminoisophthalic acid, 5-N,N-diethylaminoisophthalic acid,2,5-dihydroxyterephthalic acid, 2,5-dihydroxyisophthalic acid,2,3-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid,2,4-dihydroxyphthalic acid, 3,4-dihydroxyphthalic acid, 3-fluorophthalicacid, 5-fluoroisophthalic acid, 2-fluoroterephthalic acid,tetrafluorophthalic acid, tetrafluoroisophthalic acid,tetrafluoroterephthalic acid, 1,4-naphthalenedicarboxylic acid,1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid,2,7-naphthalenedicarboxylic acid, diphenic acid,1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, diphenyl ether4,4′-dicarboxylic acid, benzophenone-4,4′-dicarboxylic acid, diphenylsulfone 4,4′-dicarboxylic acid, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid,2,2-bis(4-carboxyphenyl)hexafluoropropane, 4,4′-stilbenedicarboxylicacid or 4-carboxycinnamic acid, or the C1-C20-alkyl esters orC5-C12-aryl esters thereof or the acid anhydrides thereof or acidchlorides thereof.
 4. The membrane according to claim 1, wherein thearomatic carboxylic acids used are tricarboxylic acids, tetracarboxylicacids or the C1-C20-alkyl esters or C5-C12-aryl esters thereof or theacid anhydrides thereof or the acid chlorides thereof.
 5. The membraneaccording to claim 1, wherein the aromatic carboxylic acids used are1,3,5-benzenetricarboxylic acid (trimesic acid);1,2,4-benzenetricarboxylic acid (trimellitic acid);(2-carboxyphenyl)iminodiacetic acid, 3,5,3′-biphenyltricarboxylic acid;3,5,4′-biphenyltricarboxylic acid or 2,4,6-pyridinetricarboxylic acid ora mixture thereof.
 6. The membrane according to claim 1, wherein thearomatic carboxylic acids used are tetracarboxylic acids, theC1-C20-alkyl esters or C5-C12-aryl esters thereof or the acid anhydridesthereof or the acid chlorides thereof.
 7. The membrane according toclaim 1, wherein the aromatic carboxylic acids used arebenzene-1,2,4,5-tetracarboxylic acids;naphthalene-1,4,5,8-tetracarboxylic acids,3,5,3′,5′-biphenyltetracarboxylic acid; benzophenonetetracarboxylicacid, 3,3′,4,4′-biphenyltetracarboxylic acid,2,2′,3,3′-biphenyltetracarboxylic acid,1,2,5,6-naphthalenetetracarboxylic acid or1,4,5,8-naphthalenetetracarboxylic acid.
 8. The membrane according toclaim 1, wherein the content of tricarboxylic acid or tetracarboxylicacids (based on the dicarboxylic acid used) is between 0 and 30 mol %.9. The membrane according to claim 4, wherein the content oftricarboxylic acid or tetracarboxylic acids (based on the dicarboxylicacid used) is between 0.5 and 10 mol %.
 10. The membrane according toclaim 1, wherein the heteroaromatic amino hydroxy carboxylic acids usedare heteroaromatic dicarboxylic acids and tricarboxylic acids andtetracarboxylic acids which comprise at least one nitrogen, oxygen,sulfur or phosphorus atom in the aromatic system.
 11. The membraneaccording to claim 1, wherein the heteroaromatic amino hydroxycarboxylic acids used are pyridine-2,5-dicarboxylic acid,pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid,pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid,3,5-pyrazoledicarboxylic acid, 2,6-pyrimidinedicarboxylic acid,2,5-pyrazinedicarboxylic acid, 2,4,6-pyridinetricarboxylic acid,benzimidazole-5,6-dicarboxylic acid, or the C1-C20-alkyl esters orC5-C12-aryl esters thereof, or acid anhydrides thereof or acid chloridesthereof.
 12. The membrane according to claim 1, wherein the aromaticand/or heteroaromatic amino hydroxy carboxylic acids used are3-amino-4-hydroxybenzoic acid or 4-amino-3-hydroxybenzoic acid.
 13. Themembrane according to claim 1, wherein, in step A), a polyphosphoricacid with a content, calculated as P₂O₅ (by acidimetric means) of atleast 79.8% is used, which corresponds to a concentration of min. 110%H₃PO₄.
 14. The membrane according to claim 1, wherein the polyoxazolepolymer comprises repeat oxazole units of the general formula (I) and/or(II) and/or (III) and/or (IV) and/or (V) and/or (VI) and/or (VII)

in which Ar is the same or different and is a tetravalent aromatic orheteroaromatic group, which is optionally mono- or polycyclic, Ar¹ isthe same or different and is a divalent aromatic or heteroaromaticgroup, which is optionally mono- or polycyclic, Ar² the same ordifferent and is a di- or trivalent aromatic or heteroaromatic group,which is optionally mono- or polycyclic, Ar³ is the same or differentand is a trivalent aromatic or heteroaromatic group, which is optionallymono- or polycyclic, Ar⁴ is the same or different and is a trivalentaromatic or heteroaromatic group, which is optionally mono- orpolycyclic, Ar⁵ is the same or different and is a tetravalent aromaticor heteroaromatic group, which is optionally mono- or polycyclic, Ar⁶ isthe same or different and is a divalent aromatic or heteroaromaticgroup, which is optionally mono- or polycyclic, Ar⁷ is the same ordifferent and is a divalent aromatic or heteroaromatic group, which isoptionally mono- or polycyclic, Ar⁸ is the same or different and is adivalent aromatic or heteroaromatic group, which is optionally mono- orpolycyclic, Ar⁹ is the same or different and is a divalent aromatic orheteroaromatic group, which is optionally mono- or polycyclic, X is thesame or different and is oxygen, and n is an integer greater than orequal to
 10. 15. The membrane according to claim 1, wherein thepolyoxazole, is a polybenzoxazole.
 16. The membrane according to claim1, wherein, in step iii), a polymer comprising repeat units of theformula

where n in each case is an integer greater than or equal to 10 isformed.
 17. The membrane according to claim 1, wherein the hydrolysis instep iv) is effected at temperatures between 0° C. and 200° C., in thepresence of humidity or water and/or water vapor.
 18. The membraneaccording to claim 1, wherein the duration of the hydrolysis in step iv)is between 10 seconds and 300 hours.
 19. The membrane according to claim1, wherein, in step iii), a layer is obtained with a thickness of 20 and4000 μm.
 20. The membrane according to claim 1, wherein the membranepresent after step iv) has a thickness between 15 and 3000 μm.
 21. Themembrane according to claim 1, wherein the hydrolysis in step iv) iseffected at temperatures between room temperature (20° C.) and 90° C.,in the presence of humidity or water and/or water vapor and the durationof the hydrolysis in step iv) is between 1 minute to 200 hours and, instep iii), layer is obtained with a thickness of between 50 and 3000 μmand the membrane present after step iv) has a thickness between 20 and1500 μm.
 22. The membrane according to claim 1, wherein the membranecomprises at least one further polymer from the group of polyazole,polysulfone, polyether sulfone or polyether ketone, where the polymersoptionally also have sulfo groups.
 23. A membrane electrode assemblycomprising at least one electrode and at least one membrane according toclaim
 1. 24. A fuel cell comprising one or more membrane electrodeassemblies according to claim
 23. 25. The membrane according to claim 1,wherein the content of polyoxazoles is between 5 and 10% by weight andthe content of phosphoric acid between 50 and 60% by weight, and theremaining amount is water.